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I asked how a navigator at sea, without internet access, would obtain
sufficiently accurate predictions of the Moon's position, other than by
using the Nautical Almanac."

Which propted this somewhat peevish response-

| First, I presume here that you are talking about predicted lunar distances
| of sufficient accuracy, right? Well, our hypothetical navigator could
easily
| PRINT THEM OUT from any of a number of sources. Isn't that just obvious? I
| mean really, REALLY obvious??

No, not at all obvious, and not so obvious to Frank that he chose to offer
that path before, when asked. Not obvious if he has no idea, at departure,
when he will get the chance to use the method , nor which stars he will
require lunar distances to, when that time comes. Presumably, that list is
to be somewhat different to the standard list of zodiacal stars, as used for
getting GMT. What time intervals between predictions will be required, to
allow interpolation to be sufficiently precise to achieve a 0.1' OVERALL
precision? Much less than Maskelyne's 3 hours, without a doubt. So just how
much information will he need to precalculate, print out, and take with him,
and what guidance will he get to produce it? Just pretending that the whole
scheme is a practical proposition; will our erstwhile lunar-navigator need
to invent something like a new nautical-almanac for himself beforehand? Far
from being obvious, it's one of the practical difficlties that Frank has
evaded, until now.

I had raised no objection to the use of a computer. Indeed, my reply to Ken
was that it is "a good way to handle the problem.". It's just that Frank,
although asked, hasn't mentioned before that to implement this trick could
require a computer on board (one that wouldn't fit on his key-chain). That,
too, has been glossed over.

How about this way of arguing. "Uh, how about we shoot some trial sights at
a known locations? That's obvious, right? And then whatever standard
deviation you get, that's what you should apply."

Not at all obvious if , for example, he wishes to compare observations made
on shipboard, with a sextant telescope appropriate for such use, with
observations made on land, with a 10x scope and a steady platform. Is his
6-mile precision intended to apply to measurement at sea, or not? But we
will have to see what his claims are, when we get some hard details, if that
day will come.

Readers can hardly fail to notice that Frank has offered us a lengthy
diatribe about errors, but once again failed to provide an error budget, or
any explanation of how he can get all the errors, systematic and random, to
combine together to within 0.1'. One more evasion.

Two things have come out of his posting. One is that there is no reliance on
data from the Nautical Almanac, which has been made clear for the first
time. The other is that his predicted error of 6 miles was at one stadard
deviation.

George.

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.


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